As a conventional fixing apparatus provided in an electrophotographic image forming apparatus, there is a roller pair type fixing apparatus made up of a fixing roller and a pressure roller.
An example of such a fixing roller is a fixing roller in which a halogen lamp is provided as a heating source inside a hollow shaft that is made of a metal such as aluminum and that has a surface on which an elastic layer is provided. Generally, a temperature of a surface of the fixing roller of this type is kept at a constant temperature as a result of ON/OFF control over the halogen lamp. The ON/OFF control is carried out by a temperature control circuit in accordance with a signal supplied from a temperature sensor provided on the surface of the fixing roller.
Meanwhile, an example of such a pressure roller is a pressure roller in which a heat resistant elastic layer such as a silicone rubber is provided as a coating layer on a shaft. When the pressure roller and the fixing roller are pressed against each other, a predetermined nip region is formed as a result of elastic deformation of the elastic layers. In the roller pair type fixing apparatus, a sheet to which an unfixed toner image has been transferred passes through the nip region, with the result that the toner image is melted by the heat and is fixed to the sheet.
However, such a conventional and general roller pair type fixing apparatus suffers from the following problem. That is, in cases where a plurality of sheets continuously pass therethrough at a fast sheet passing speed, the temperature of the surface of the fixing roller is extremely decreased. This makes it difficult to keep the temperature of the surface of the fixing roller at a constant temperature. A reason of this is that: the heat generated from the inside of the shaft is conducted to the toner image via the elastic layer (e.g., silicone rubber), which has a bad heat conductivity, and it therefore takes a quite a long time that the heat is conducted from (i) the heating source provided inside the shaft to (ii) the surface of the fixing roller.
In order to solve such a problem, an external heating type fixing apparatus has been proposed. Such an external heating type fixing apparatus is disclosed in, e.g., Patent Citation 1 (Japanese Unexamined Patent Publication Tokukai 2005-266395 (published on Sep. 29, 2005)) and Patent Citation 2 (Japanese Unexamined Patent Publication Tokukai 2005-292714 (published on Oct. 20, 2005)). In the fixing apparatus disclosed in each of Patent Citations 1 and 2, an endless belt (external heating belt) is set around a plurality of belt supporting rollers (external heating rollers). The endless belt is heated and is brought into contact with a fixing roller, thereby heating a surface of the fixing roller.
In the meanwhile, Patent Citation 3 (Japanese Patent Publication Tokkyo 3632724 (publication date of Laid-Open stage is Jan. 29, 1999)) discloses a fixing apparatus including a releasing/contacting mechanism for bringing an external heating member out of contact with a fixing roller (heating roller). This prevents the fixing roller from leaving a trace on the external heating member as a result of being pressed against the external heating member. Further, Patent Citation 3 describes that the external heating member is pressed against the fixing roller when the external heating member is in a warm-up state or stand-by mode and reaches a reference temperature equal to or higher than a toner softening point. This prevents the surface of the fixing roller from being scratched by toner transferred from the fixing roller to the external heating member and adhered to and accumulated in the external heating member.
However, each technique of Patent Citations 1 and 2 suffers from the following problem. Consider a case where the endless belt is left for a long time with no rotation of the fixing roller. In this case, the endless belt will remain in a deformed shape in which the endless belt has been while the endless belt is left. For example, in cases where the endless belt is set around two belt supporting rollers, the endless belt will remain in an elliptic shape (keeps on being bent). The endless belt thus remaining in such a deformed shape causes a trouble in rotation, when the fixing roller is started to rotate. A specific example of the trouble is slip or the like. Especially, in cases where the endless belt is left at a high temperature for a long time and is thereafter cooled down to a room temperature, the endless belt will be highly likely to remain in such a deformed shape.
The following explains this problem more in detail. In cases where an external heating device is constituted by an endless belt held by a plurality of belt supporting rollers as is the case with Patent Citation 1 and 2, each of the belt supporting rollers normally has a small diameter so as to have a heat capacity as small as possible. Thus, the endless belt is held by such belt supporting rollers each having a small curvature radius. In cases where the endless belt is left for a long time with no rotation of the belt supporting rollers, the endless belt will remain in a deformed shape that the endless belt has while the endless belt is left. Usually, for attainment of the small heat capacity, the number of the belt supporting rollers is two, which is minimally required number. The endless belt thus set around the two belt supporting rollers is tensed between the two belt supporting rollers, so that the endless belt will remain in an elliptic shape especially in cases where the endless belt is left for a long time with no rotation of the belt supporting rollers.
When starting to rotate the endless belt thus remaining in such a deformed shape, a rotation trouble of the belt such as slip occurs due to the deformed shape. In addition, such a rotation trouble as slip of the belt is especially likely to occur in cases where a material having a small friction resistance, such as a PFA tube, PTFE, or PFA coating, is used for respective surfaces of the fixing member and the belt. Such a material having a small friction resistance is frequently used therefor in view of a releasing property.
When the rotation trouble of the endless belt occurs, the surface of the fixing roller and the surface of the endless belt are rubbed with each other, with the result that the surface of the fixing roller is scratched by toner accumulated, due to offset or the like, on either the surface of the fixing roller or the surface of the endless belt.
In addition, in cases where there is a restriction in a distance between the respective axes of the belt supporting rollers, slip shaft bearings are likely to be used as supporting members respectively provided in the ends of each of the belt supporting rollers, instead of ball bearings. Further, for simplicity of mechanism and space saving, the fixing apparatus is possibly arranged such that: the external heating device is not provided with a driving source, and the endless belt is rotated according to the rotation of the fixing roller. In other words, the fixing apparatus is possibly arranged as follows. A rotation driving source is provided in the fixing roller, and the endless belt is brought into contact with the surface of the fixing roller such that the endless belt and the belt supporting rollers are rotated due to friction with the fixing roller. In these cases, the aforementioned rotation trouble of the endless belt especially occurs with ease.
Meanwhile, in Patent Citation 3, during the warm-up or the standby mode, the external heating member is brought out of contact with the fixing roller. With this, the external heating member is not pressed against the fixing roller, and no trace of the fixing roller is therefore left on the external heating member. However, even though the external heating member is brought out of contact with the fixing member as such, it is impossible to prevent the endless belt from remaining in the deformed shape after being left for a long time, in cases where the external heating device in which the endless belt is set around the belt supporting rollers is used.
Further, in Patent Citation 3, during the warm-up or the standby mode, the external heating member is pressed against the fixing roller when the external heating member reaches the reference temperature equal to or higher than the toner softening point. This softens the toner adhered to the surface of external heating member. Therefore, even though the external heating member is pressed against the fixing roller, the fixing roller is never scratched by the toner adhered to the external heating member. Patent Citation 3 thus makes it possible to soften the toner adhered to the external heating member, but does not take into consideration the external heating device in which the endless belt is set around the plurality of belt supporting rollers. Therefore, Patent Citation 3 possibly cannot prevent the rotation trouble occurring due to the deformed shape in which the endless belt remains.
Further, the external heating device generally has a small heat capacity such that the temperature thereof is raised quickly. This makes it difficult to carry out precise temperature control when the rotation trouble occurs. For example, in cases where temperature control is started when a temperature sensor or the like detects that the temperature of the external heating device reaches the set temperature, the temperature thereof becomes very high due to overshoot. This is problematic. When the external heating device has too a high temperature, the heat causes problems such as (i) breakage of the endless belt or the coating layer of the fixing roller and (ii) twisting of the belt supporting rollers.